A case report is presented of a large retroperitoneal hibernoma along with the clinical, radiographic, and pathologic findings. Despite the unusual size and location of this hibernoma, the case illustrates the salient features of these rare tumors of brown fat origin. Pitfalls in preoperative diagnosis are discussed. To date, there has been no convincing evidence of malignant potential of this tumor, and total excision remains the treatment of choice.

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